1. Field of the Invention
The present invention relates to a throttle valve used in an air flow control assembly for adjusting the amount of air flowing through an air intake passage of an internal combustion engine.
2. Description of the Related Art
An example of a conventional air flow control assembly for an internal combustion engine is shown in FIG. 12.
In the figure, a generally cylindrical rod-shaped throttle shaft 12 is rotatably supported at both ends by a throttle body 6 having a generally cylindrical inner wall 6a. A long, thin rectangular groove is disposed in the throttle shaft 12, and a disk-shaped throttle valve 1 composed of injection-molded resin is inserted into the groove and secured to the throttle shaft 12 by screws (not shown). The throttle valve 1 rotates together with the throttle shaft 12 and adjusts the amount of air flowing into a combustion chamber (not shown) of the internal combustion engine by changing a gap G between the throttle valve 1 and the inner wall 6a of the throttle body 6.
Next, the method for manufacturing this throttle valve 1 by injection molding will be explained.
FIG. 10 is a cross section explaining the process of injection molding the throttle valve 1 by injecting a resin 5 into an injection mold 10 in the shape of the throttle valve 1.
In the figure, a flat disk-shaped cavity 11 for forming the throttle valve 1 is disposed in the injection mold 10. An injection mold gate portion 2, which is a small-diameter cylindrical opening for injecting the resin 5 into the cavity 11, is disposed on one side of the center of the cavity 11, and the resin 5 is injected from a runner 3, which is a large-diameter cylindrical opening, through the injection mold gate portion 2 to fill the inside of the cavity 11.
FIG. 11 is a plan showing the resin 5 as it fills the inside of the cavity 11 of the injection mold 10.
In the figure, the resin 5 is injected into the cavity 11 through the injection mold gate portion 2 and spreads radially from there. Cylindrical pins 16 are disposed inside the cavity 11 to form shaft-securing bores for passage of the screws securing the throttle valve 1 to the throttle shaft 12.
To permit the engine to perform stable low-fuel-consumption idling, it is necessary to suppress the amount of air leakage, which is the amount of inflow air leaking through the gap G, when the throttle valve 1 is fully closed, that is, the state in which the throttle valve 1 has rotated to be perpendicular to the inner wall 6a.
However, since the precision of outside diameter dimensions of the throttle valve 1 has been insufficient, the gap G could not be adequately reduced, increasing the air leakage when the throttle valve 1 is fully closed, and making it difficult to achieve stable low-fuel-consumption idling.
It is necessary to improve precision of the outside diameter of the throttle valve 1 to reduce air leakage, but as shown in FIG. 11, since there is no means provided for controlling the flow of the resin 5 injected into the cavity 11 through the injection mold gate portion 2, the speed of the resin 5 flowing through the cavity 11 is not uniform. For that reason, the time at which the resin 5 reaches different positions on the outside diameter mold portion 15, which forms the outside diameter portion of the throttle valve 1, is irregular, making the density to which the resin 5 fills the inside of the cavity 11 nonuniform due to irregularities in the hardening time of the resin 5, thereby leading to deterioration in the precision of the outside diameter dimensions, which includes the outside diameter dimensions, outside diameter roundness, etc., of the throttle valve 1. Conventionally, the irregularities in the outside diameter dimensions of injection-molded throttle valves 1 are in the order of one percent of the outside diameter dimensions, which is approximately ten times the irregularities in outside diameter dimensions of generally-used conventional throttle valves in which metal material is machined.
In addition, when the cross-sectional shape is asymmetrical as in the throttle valve 21 shown in FIG. 13, the time at which the resin 5 reaches different positions on an outside diameter mold portion 22 of the throttle valve 21 is even more irregular, and the deterioration in the precision of the outside diameter dimensions has been significant.
Furthermore, as shown in FIG. 11, during the process of injecting the resin 5, the resin 5 which has started to harden and build up in the vicinity of the injection mold gate portion 2 flows radially outwards through the cavity 11, but this partially-hardened resin is resistant and causes the flow of the resin 5 being injected into the cavity 11 after it to be non-uniform, leading to deterioration in the precision of the outside diameter dimensions of the throttle valve 21.
FIG. 14 is a cross section showing a throttle valve 1 manufactured by injecting the resin 5 into the injection mold 10 shown in FIG. 10.
In the figure, some of the resin from the injection mold gate portion 2 of the injection mold 10 has been left behind and formed a burr 13 at a gate resin portion 17 in the center of the throttle valve 1 where the resin was injected during injection molding. Such burrs 13 catch on the groove in the throttle shaft 12 when the throttle valve 1 is being inserted into and secured to the groove in the throttle shaft 12, and it has not been easy to install the throttle valve 1 in the throttle shaft 12.